1. Field of the Invention
The present invention relates to a stepping motor control circuit that controls the driving of a stepping motor, a movement including the stepping motor control circuit, and an analog electronic timepiece including the movement.
2. Background Art
Hitherto, in analog electronic timepieces, a drive system has been developed in which, when a plurality of main drive pulses P1 are prepared and a stepping motor is driven by any of the main drive pulses P1, pulses are controlled so that a change (rank-down) to the main drive pulse P1 having small energy is performed in a case where energy of the main drive pulse P1 has a drive margin, and a change (rank-up) to the main drive pulse P1 having large energy is performed in a case where energy of the main drive pulse P1 has no drive margin, thereby allowing the stepping motor to be stably rotationally driven and allowing electric power saving to be achieved.
Drive systems are developed in which at the time of detecting whether the energy of the main drive pulse P1 has a margin, a detection section for detecting the rotation of a stepping motor is divided into a plurality of sections by a predetermined reference time, the degree of the drive margin of the main drive pulse P1 is determined on the basis of a section in which an induced signal VRs exceeding a predetermined reference threshold voltage generated by the free vibration of the stepping motor is detected, and driving is performed by a change to the main drive pulse P1 depending on the degree of the drive margin (see, for example, International Publication No. WO 2005/119377 and JP-A-2010-166798).
In the drive systems disclosed in International Publication No. WO 2005/119377 and JP-A-2010-166798, since the reference time for setting the section is constantly fixed, rank-up (excess rank-up) is performed when the detection time of the induced signal VRs becomes late in spite of the drive margin being present. For example, in a case where the moment of hands mounted to an analog electronic timepiece by a customer is larger than specified, or the like, the timing of the generation of an induced voltage occurring due to free vibration is delayed, and thus the induced signal VRs indicating that rank-up is required is generated. Excess rank-up may occur in spite of rank-up being unnecessary or rank-down being necessary. Due to the occurrence of excess rank-up, current consumption increases, and thus battery life deteriorates when a battery is used. Thereby, there occurs a problem in that battery life considerably fluctuates or deteriorates because of the moment of hands to be mounted.